Certain “direct impingement” or “external piston” gas operated rifles, i.e. semi-automatic or automatic rifles, use a barrel with a small hole or gas port that goes from the bore to the exterior. As the rifle is fired, gas flow is diverted through the gas port and is then channeled through a gas block, into either a gas tube or external piston which then transfers energy back into the bolt group in order to cycle the firearm for the next round. In certain situations it is desirable to control the amount of gas which can pass through the hole in the barrel to provide more or less force. For example, less force may be desired if a suppressor is used with the firearm, whereas more force may be desired if there is a build up of shot residue in the system. One method of controlling this pressure uses a valve attached to the return gas tube.